Working machinery is a major source of vibration in marine vessels and considerable effort is devoted to developing isolation systems that reduce transmission to the hull. A particular problem associated with machinery isolation in marine environments is structural resonance. This occurs principally in the machinery support structure. Such resonance leads to very high forces transmitted across machinery mounts, and this poses a very significant vibration problem. Moving machinery generates a complex spectrum of out-of-balance forces and in marine vessels considerable effort is devoted to developing resilient mounting systems that reduce the transmission of these forces across the machinery mounts to the hull.
A common approach to vibration isolation is to mount marine machinery items on a framework or raft and to support this raft from the hull on a set of rubber mounts. If the supported structures behaved as an ideal rigid body, the force transmissibility curve (transfer function from vibration force to transmitted force) would be as the monotonically descending line of FIG. 1. In practice however the supported machinery and its raft will always be flexible to some degree. As a result, structural resonances are excited, and a typical force transmissibility curve for resilient mounts is shown by the peaked curve in FIG. 1. This illustrates three distinct frequency regimes: the first below the 5 Hz resonance, where the entire force generated by the machinery, primarily the gravitational force, is transmitted through the resilient mounts; the second, the 5 Hz resonance itself where the machinery, acting as a rigid body, is “bouncing” on the resilient mounts, and the third, above the 5 Hz resonance, where the machinery is becoming flexible and individual structural resonant modes are excited. The 5 Hz resonance is called the mount resonant frequency. Above this frequency the force transmissibility is generally decreasing with increasing frequency and this results in forces generated by machinery vibrations being attenuated before arriving at the hull. However, the force attenuation in this regime may be dominated by structural resonances. Structural resonances act as mechanical amplifiers and hence generate large peaks in the force transmissibility curve as shown.
Because of the compromises that have to be made in designing passive isolation systems, active and semi-active systems have been proposed. In Patent Application WO 01/18416, and Daley, S., et al, Active vibration control for marine applications, IFAC Journal Control Engineering Practice, Volume 12, Number 4, pp 465-474, published 25 Jul. 2003, and in Johnson, A. and Daley, S., A Smart Spring Mounting System for Marine Applications, ISCV11 Conference on Sound and vibration, St Petersburg, July, 2004, an active mount system is proposed comprising an array of a large number of mounts, each mount comprising an electromagnetic actuator in parallel with passive elements to form a composite mount as shown schematically in FIG. 2. In order to avoid transmission of large forces at frequencies corresponding to supported structure resonances, the mounting system fulfils a number of key requirements. The first requirement is for the composite mount not to transmit any additional force to the hull as a result of any local displacement of the supported structure at its attachment point. As a result no additional force is generated on the hull from excited resonances. Thus the composite mount must have effectively zero stiffness. A second requirement is that to support the structure each composite mount must also be able to generate an external demand force for compensating for out of balance forces. Out-of-balance forces, generated by the moving machinery, result in both linear and angular displacements of the supported structure. The external demand forces generated by each composite mount are the means whereby these linear and angular displacements can be continuously opposed to return them towards their equilibrium positions in a controlled way. As shown in FIG. 2, the actual force on a hull generated by the composite mount is measured by a load cell (or strain gauge) and compared with a global demand value, in order continuously to correct the current of the electromagnet.
In use, each electromagnet is first used to pre-stress the passive mount elements by a static force F so that the change in the force on the machinery may be ±F by increasing or decreasing the current through the electromagnets; thus a maximum control force of at least 2F is required to be generated. When the power is switched off this pre-stress is relieved. A difficulty with this simple approach is that the large non-linearity of the electromagnet makes a simple feedback control unsatisfactory. To overcome this, a more complex local control is needed involving both feed-forward of the relative mount displacement and feedback of the transmitted force
Further improvements in mounting systems are desirable, in particular for reducing complexity and size of the mounting system.